Illustrated herein in embodiments are liquid crystal displays, more specifically, bistable cholesteric liquid crystal displays (LCDs). The displays exhibit enhanced white reflectivity among other characteristics.
The cholesteric liquid crystal display has attracted attention in recent years as an electronic paper (e-paper) type of display device. The cholesteric liquid crystal display is capable of utilizing reflection from surrounding lights as a light source and has a storage property which can hold display contents after the supply voltage is turned off. Further, because an active matrix is not needed for driving the display, cholesteric liquid crystal display devices are capable of providing cost effective large-capacity displays, and may use a flexible substrate which is particularly suitable for e-paper.
A procedure for fabrication of Domain Controlled Liquid Crystal Display providing a bistable display has been described previously by G. Iftime et al., in U.S. Pat. Nos. 6,767,480 B2, 6,824,708 B2 and US Pat. Appl. Publ. (2004) US 2004115366A1, the entire contents of which are incorporated herein by reference. The size of the liquid crystal domains is controlled with an electric field. Consequently, the displays can switch between smaller size domains which strongly scatter the ambient light, i.e. display appears white, and larger sized domains which weakly scatter the ambient light, i.e. display appears transparent. In the transparent state, a viewer sees the color of the background, for example black.
Additionally, a white reflecting cholesteric liquid crystal may be made bistable by the addition of a dipolar dopant into the liquid crystal mixture. However, in order to further increase the reflectivity in the white state, it is necessary to further decrease the size of the smaller size liquid crystal domains. This can be achieved if all the liquid crystal domains are generated of a smaller size, including both of the smaller and larger sized domains in the liquid crystal mixture. This can also be produced by placing the liquid crystal mixture containing a liquid crystal domain stabilizing material into a polymer network, oriented perpendicularly to the substrates. Fixed maximal size of liquid crystal domains can be obtained by phase separation of insoluble polymer network inside the cell.
A procedure for fabricating fixed-size liquid crystal domains has been described previously by J. W. Doane et al. in U.S. Pat. No. 5,691,795, and the procedure was used for fabrication of normal mode, light modulating, polymer stabilized liquid crystal displays (PDLCs). The procedure described involves ultraviolet (UV) initiated polymerization of a monomer dissolved into a cholesteric liquid crystal, which is in a homeotropic state. In the homeotropic state, the molecules of liquid crystal are orthogonally aligned to the cell walls. The homeotropically aligned liquid crystal acts as a template for formation of polymer networks orthogonally aligned to the cell walls.
The monomer contains at least two polymerizable groups capable of producing a wire-like polymer. In previous art, the polymer concentration was tuned to a level suitable for generation of physically separated liquid crystalline domains of about 1 μm to about to 4 μm. At this size, the liquid crystalline domains are strongly scattering, but their size is fixed. This type of device is not bistable, requiring an externally applied electric field to maintain a particular state. Transparent state is achieved by applying an electric field high enough to reach the homeotropic state, but when the electric field is removed, it relaxes back to the strongly scattering state.
Therefore, there is a need for an economical method of making a cholesteric display which is bistable and has improved white reflectivity over previous designs.